According to a known electric power steering device for motor vehicles, the motor shaft consists of a hollow shaft, and a drive shaft connected to the right and left steerable wheels via tie rods is coaxially received in the motor shaft. A ball nut arrangement is interposed between the hollow motor shaft and the drive shaft for converting the rotational movement of the hollow motor shaft into the axial movement of the drive shaft.
FIG. 3 illustrates a conventional electric power steering device which comprises a yoke 1 consisting of an axially elongated tube member serving also as a part of the housing for the power steering device, and a gear casing which accommodates a rack and pinion gear mechanism and coaxially attached to the yoke 1 via a radial flange to form the rest of the housing for the power steering device.
The yoke 1 and the gear casing 2 thus jointly form the housing for the power steering device, and coaxially receive therein a drive shaft 3 which is axially slidable so as to steer the right and left front wheels via the tie rods attached to either axial end thereof. The drive shaft 3 is connected to the rack of a rack and pinion gear device, and the pinion thereof is attached to a pinion shaft 4 which is connected to a steering shaft.
An intermediate part of the drive shaft 3 is coaxially received in a hollow motor shaft 5. A motor armature core 6a and a commutator 6b are attached to the outer circumferential surface of the hollow motor shaft 5 so that the armature core 6a and the hollow motor shaft 5 rotate integrally together. A ball nut mechanism is interposed between the inner circumferential surface of the hollow motor shaft 5 and the outer circumferential surface of the drive shaft 3 to convert the rotational movement of the hollow motor shaft 5 to the axial movement of the drive shaft 3. The rotational torque produced from the motor armature 6 is thus converted into the axial assist force for the drive shaft 3 so that the effort required for turning the steering wheel attached to the pinion shaft 4 may be reduced.
The power steering device normally extends laterally across the vehicle body which is not shown in the drawing, and is attached to the vehicle body via a bracket 2a formed in the gear casing 2 and a mounting bracket 13 partly surrounding a small diameter portion 1c of the yoke 1.
These brackets 2a and 13 are normally secured to the vehicle body by fasteners such as threaded bolts. The main part 1a of the yoke surrounding the motor armature 6a is necessarily larger in diameter than the small diameter portion 1c of the yoke 1 which is to be mounted to the vehicle body via the bracket 13, and it is necessary to connect the small diameter portion with the rest of the yoke with a tapered section 1d.